For the level measurement in an industrial environment, highly widely-varied physical principals are currently being used. Factors such as the corrosivity or the flammability of the substances to be measured or the extreme pressure and temperature conditions will have a bearing on the type of measuring device selected, as well as on the precision, reliability and physical limitations of the system. In the aforementioned selection of the type of measuring device, the static measuring devices are preferred over those composed of moving parts and those not requiring any contact with the fluid or which are even external to the system. However, industry has focused its efforts on level reading in large containers.
Although there are a great many operating principles upon which the different level measuring devices currently on the market are based (flotability, ultrasound, conductivity, laser, differential pressure, resistivity, capacity, microwaves, radioactivity, deformation, etc.), in the environment of a pilot micro-plant or laboratory reactor scale, basically any type of instrument for level measuring is ruled out as a result of the installation-related limitation stemming from the size of the instruments in some cases and from the measurement range intended to be achieved in others. The only valid method for the measurement thereof in these systems is the indirect measurement by means of the differential pressure existing between the top and bottom of a container which results from the pressure exerted on the base thereof by the hydrostatic column:P=ρ·g·h 
Focusing the current situation of the available technology in the application thereof to the type of systems being dealt with herein, it can be said that there is no level-measuring instrument on the market which can satisfactorily provide this measurement given the size-related specifications the control thereof requires in a continuously-operating laboratory reactor.
Thus, for example, a container where condensation of the liquid reaction byproducts takes place at the outlet of a reactor must avail itself of a continuos level measurement for the purpose of adjusting the outflow of liquids from the reactor, a phenomenon which occurs at high pressures in this container, i.e., on the order of 100 bar. Condensation occurs continuously, and if prevention of the accumulation of these products is intended for the purpose of making the analysis of samples in certain reaction times possible, the situation should be as close as possible to “drop that condenses, drop that is evacuated form the system”. To this end, it is absolutely essential to keep a constant level, which also serves as a hydraulic seal of the system.
There are currently different builders of these types of systems that use the pressure differential measurement in the container as an indirect level measurement thereof. Generally speaking, this is a matter of two branches, an upper and a lower branch, which capture the different pressure between the ends of the container. But with this system, the fluid column must be kept several centimeters in height above the high pressure-reading branch due to the precision of these systems and to their zero error, choosing an insufficient height could be critical if the zero error were to mask the transmitter signal and the control system were to receive a wrong signal. Were this phenomenon to occur, the control valve which regulates the height of this fluid column would tend to open until it evacuated the liquid from the container, at which point the hydraulic seal would be lost, and the gasses from the system would flow to the outside, possibly causing an accident due to their toxicity or flammability.
If one adds to the above the liquid column which must fill the branch which transmits the pressure to the measuring device and the amount of liquid which must fill the chamber of the reading instrument, a design with a considerable dead volume would have to be made, which, in a situation in which the system is supplied with a small flow rate of liquids, on the order of 0.01 ml/min, it would take a considerable length of time, even several hours, to achieve the first drop of sample of liquid at the outlet.
This is avoided by keeping a liquid inundating the system constantly, which contaminates the sample. In any event, it is inevitable that the sample at the outlet be a measurement of the products collected throughout a long period of time. The situation is not critical at large-scale pilot plants, where the flow rates are considerably higher and these effects are negligible, but it is definitively unacceptable in reactors which work with very small volumes, such as is the case, for example, of the catalytic microactivity studies, pilot plants under supercritical conditions with backflow column, agitated autoclaves of volumes ranging from 50 to 1000 ml, and other small-volume systems.